This invention relates primarily to luminaires for ceiling mounting, whether on the ceiling surface, suspended therefrom or recessed therein. It also relates to units emitting luminous flux upward for the purpose of reflecting light off a surface above.
For optimum lighting efficiency and architecturally pleasing appearance, lighting fixtures mounted in or on the ceiling have most often been utilized, especially when the area to be lighted is large. Such areas include open spaces in business offices, department stores, grocery stores, classrooms, warehouses, corridors, and the like. While luminaires incorporating one or more parabolic reflecting surfaces have been used to reflect or redirect light in order to provide efficient distribution of light flux, photometrically illustrated as half bat-wing or full bat-wing shaped areas defined by photometric curves showing relative candlepower distribution, their efficiency has not been fully utilized due to veiling reflections and direct discomfort glare especially resulting when the light source includes a light diffusing envelope enclosing a light emitter (i.e., phosphor coated lamps such as, but not limited to, mercury, fluorescent or metal halide.)
Veiling reflections are the reflections of a light source in the visual task, (e.g., the printed page) that cause a reduction in luminance contrast (e.g., between the print and its background). Since the normal sight-line of a worker, in performing a desk-type visual task, is downward in a line about 25.degree. to the worker's side of a line perpendicular to the plane of the task, a light source which is positioned approximately in a line 25.degree. behind the line perpendicular to the plane of the task, will be reflected by the task into the eyes of the worker. The black part of a visual task (such as print) approaches the brightness of the white part (such as paper) and thus a reduction of luminance contrast results, reducing visibility. The reflections therefore act as a veil placed on the visual task, hence, the term, veiling reflections.
The deficiency results especially from improper distribution of light flux in the work surface zone, that is, the zone of about 0.degree. to about 25.degree. above nadir. This zone is the normal sitting viewing zone of an office worker at his desk. Even though exsisting ceiling fixtures utilize one or more parabolic reflectors for more efficient candlepower distribution and minimization of veiling reflections, some veiling reflections nevertheless are apparent from a task in the 0.degree. - 25.degree. zone when the light source has a light diffusing envelope.
Study of the problem has revealed that the veiling reflections and direct glare (at normal viewing angles 45.degree. - 90.degree. from nadir) result from either improper placement of the parabolic reflectors or surface relative to the light source, or the light source having light diffusing envelopes which provide light emanations from near tangential on all surfaces of the light diffusing envelope of the light source. Accordingly, a significant improvement in the utilization of parabolic reflectors in ceiling lighting fixtures will result if the parabolic reflectors can be arranged, and other means provided, to eliminate the effects of the nearly tangential light emanations in the zone of about 45.degree. to 90.degree. and 0.degree. to about 25.degree. above nadir.
Comparable considerations apply to the mounting of a luminaire inverted to illuminate the ceiling from a position below standing eye level or from a higher position (when desired) in order that the brightness of the ceiling directly over the luminaire, which might otherwise be excessive, may be ameliorated by the control inherent in this new design. (References herein to "above nadir" should be read as "below zenith" when referring to the luminaire in inverted position.